The present invention relates to a semiconductor laser device and an optical pickup device for makeup of, for example, optical disk systems and, more particularly, relates to a semiconductor laser device equipped with a semiconductor laser in which the plane of polarization of an outputted laser beam is inclined with respect to the substrate.
FIG. 5 shows a conventional semiconductor laser device 100 as viewed downward along a direction vertical to the drawing sheet that is the outgoing direction of the laser beam. The semiconductor laser device 100 shown in FIG. 5 is in a state with the cap removed for explanation's sake.
In this semiconductor laser device 100, a semiconductor laser 104 is die bonded to a side face 102A of a heat radiation base 102 protruding upward from a metallic stem body 101. The side face 102A, on which the semiconductor laser 104 is mounted, is referred to as die bond surface. In FIG. 5, reference numeral 105 denotes a photodetection device 105 for monitoring of the power of the semiconductor laser 104, 107 denotes a metallic wire 107, and 108 denotes a lead pin fixed to the stem body 101 via a dielectric. Also, numeral 106 denotes a photodetection device for use of signal detection, having a plurality of photo-detecting portions and signal processing circuits integrated therein.
This signal-detection use photodetection device 106 detects a signal beam reflected by an optical disk to read a signal from the optical disk or detect a focal error signal or track error signal.
The die bond surface 102A is, normally, set parallel to a reference plane 103 which is part of an outer peripheral surface 101A of the stem body 101. The outer peripheral surface 101A is composed of a parallel surface 111 opposite to the reference plane 103, and circular-arc surfaces 112, 113 which connect the reference plane 103 and the parallel surface 111 to each other.
Normally, these circular-arc surfaces 112, 113 have their centers placed at the emission point of the semiconductor laser 104. According to this placement, rotating the semiconductor laser device 100 about the emission point along cylindrical surfaces which are extension surfaces of the circular-arc surfaces 112, 113 of the stem body 101 makes it possible to adjust any inclination of an outgoing laser beam extending from the emission point about the optical axis without moving the emission point of the semiconductor laser 104.
Next, FIG. 6 shows the structure of an optical pickup device 200 equipped with the semiconductor laser device 100. This optical pickup device 200 is made up of the semiconductor laser device 100, a prism 203, a photodetector 204 for monitoring of laser power, a collimator lens 202, a raising mirror 214, and an objective lens 305. In FIG. 6, a one-dot chain line J represents the optical axis of a laser beam emitted from the semiconductor laser 104 of the semiconductor laser device 100.
The laser beam emitted from the semiconductor laser 104 is outputted in parallel to the reference plane (not shown) of the optical pickup device 200. This laser beam is partly branched by the branching use prism 203, coming incident on the laser-power monitoring photodetector 204.
In this semiconductor laser device 100, as shown in FIG. 5, a laser beam emitted from the rear face of the semiconductor laser 104 is received by the photodetection device 105, by which the laser power is monitored. Otherwise, as shown in FIG. 6, a laser beam emitted from the front face of the semiconductor laser 104 may also be monitored by the photodetector 204. This photodetector 204 is used in cases, for example, where the semiconductor laser 104 is a high-power laser with a small output from the rear face of the semiconductor laser 104 or where a high-density optical disk system to which high monitor precision is required is built up.
The laser beam transmitted by the branching prism 203 is collimated by the collimator lens 202 into parallel light, thereafter turned in direction by the raising mirror 214, and condensed to an information recording surface of an optical disk 306, which is an optical information recording medium, by the objective lens 305. The signal light reflected by the optical disk 306 travels along the original route indicated by the one-dot chain line, and is partly branched by a hologram device (not shown) integrated with the semiconductor laser device 100, and comes incident on the signal-detection use photodetection device 106. Thus, the signal from the optical disk 306 is read.
The reference plane 103 of the semiconductor laser device 100 is a reference plane for determining the position of the semiconductor laser device 100 with respect to the reference plane (not shown) of the optical pickup device 200. Normally, this reference plane 103 is positioned so as to be parallel to the mounting surface, and slightly rotated and adjusted around the optical axis J of the laser beam.
Accordingly, the thickness D of the semiconductor laser device 100 is determined by a distance from the reference plane 103 to the semiconductor laser 104, and reducing the thickness D causes the semiconductor laser device 100 to be thinned, allowing the optical pickup device 200 to be also thinned. Further, the smaller the distance between the optical disk 306 and the reference plane of the optical pickup device 200 is, the thinner the optical disk system can be made.
Also, as illustrated in FIG. 3, the laser beam of the semiconductor laser 104 expands as it travels along the optical axis J, where the expansion is formed into an increasingly widening conical shape with the optical axis J serving as a central axis. This increasingly widening conical shape has an elliptical-shaped outer circumference. The major axis of this ellipse is vertical (or parallel) to a substrate surface 104A of the semiconductor laser 104. The minor axis of the ellipse is parallel (or vertical) to the substrate surface 104A. Also, the substrate surface 104A is parallel to the die bond surface 102A and the reference plane 103.
Meanwhile, conventionally, in the semiconductor laser 104, the direction of polarization of a laser beam would be the same as the direction of expansion of the laser beam. Therefore, it has been the case that considerations are given not to the direction of polarization of the laser beam, but to the direction of expansion of the laser beam alone. That is, it has conventionally been practiced that the direction of the major axis in the expanding ellipse of the laser beam is set vertical (or parallel) to the reference plane 103 of the stem body 101.
For example, in one concrete example shown in FIG. 3, the major axis of the expanding ellipse of the laser beam is a Y axis vertical to the reference plane 103 and its minor axis is an X axis parallel to the reference plane 103. Also in this one example, the direction of polarization of the laser beam is along the X axis, while the direction of polarization is along the Y axis.
In contrast to this, it is a recent trend that a semiconductor laser in which the direction of expansion or direction of polarization of the laser beam is inclined from the horizontal or vertical direction with respect to the substrate surface of the semiconductor laser is employed with a view to improving the semiconductor laser characteristics (e.g., for higher power).
For instance, when a semiconductor laser in which the direction of polarization is inclined by a specified angle θ from the substrate surface is die bonded to a die bond surface 102A of the conventional stem body 101, the direction of the minor axis in the intensity distribution and the direction of polarization of an output laser beam would be inclined by the specified angle θ from the X-axis direction parallel to the substrate surface.
In the case where such a semiconductor laser device is used as a light source for a conventional optical pickup device, the branching ratio of the branching prism 203 or the branching ratio of the hologram device would deviate from design values, posing an issue that specified characteristics could no longer be obtained. The reason why the specified characteristics could no longer be obtained is due to the fact that reflection characteristics and transmission characteristics of a dielectric multilayer film that makes the optical branching prism 203 has a polarization dependency. Also, in holograms or other like optical devices in which grooves are arrayed in a fixed direction, their optical branch characteristics would change depending on the relationship between the direction of grooves and the direction of polarization.
In the semiconductor laser device 100 shown in FIG. 5, the outer peripheral surface 101A of its stem body 101 is formed into a shape that a circle including circular-arc surfaces 112, 113 is cut by a chord making the reference plane 103, so that the distance from the reference plane 103 to the optical axis J is reduced, thus achieving the thinning of the device.
In contrast to this, in the case where the semiconductor laser device 100 has a semiconductor laser whose direction of polarization is inclined by a specified angle θ from the substrate surface, the semiconductor laser device 100 is rotated about the optical axis J with respect to the optical pickup device 200 by a specified angle (−θ) inverse to the specified angle θ so that the direction of polarization of a laser beam of the semiconductor laser is parallelized to the reference plane of the optical pickup device 200.
With the placement that the semiconductor laser device 100 is rotated relative to the optical pickup device 200 as shown above, the optical axis J and the reference plane 103 of the semiconductor laser device 100 are inclined with respect to the reference plane of the optical pickup device 200, which would cause an obstacle to the thinning of the optical pickup device, as an issue.